The present invention relates to antennas and more precisely to antennas for transceivers of the portable telephone type.
Portable telephones, designed to be used as terminals of mobile networks or to communicate with a fixed terminal, include an antenna which is fixed to the casing of the telephone and which is used to transmit radio messages to or from the portable telephone. In conventional use, the portable telephone is near the mouth and the ear. The user""s head intercepts some of the radiation that could be beneficial to the quality of the radio link; also, electrical currents flow in the casing of the telephone and in the antenna and the proximity of the casing to the head means that these currents are absorbed or induce radiation in their environment which is wasted for the radio link. Earlier proposed solutions usually relate to the antenna and not to the currents which flow in the casing, even though they can represent the greater part of the energy wasted.
From the point of view of the function of the portable telephone, which is to provide efficient radio transmission, the portion of the electromagnetic energy absorbed by the user is wasted. The radiation lost for transmission is dissipated in the form of heat.
EP-A-0 588 365 describes a portable telephone in which the antenna radiation pattern is essentially on the side remote from the user""s head. That document proposes placing the antenna in a position on the casing which is as far as possible from the user""s head, and choosing the orientation of the antenna so as to limit radiation into the user""s head. The casing can then serve as an electromagnetic screen. It is also proposed to mount the antenna on a telescopic arm which does not radiate and which is hinged to the casing. The arm can contain a coaxial cable or can itself form a coaxial cable.
DE-A-195 02 652 proposes to dispose the antenna of a portable telephone at one end of a telescopic support which does not radiate. It also proposes to dispose the antenna at one end of a support which does not radiate and which is pivoted to the upper end of the side of the casing. When not in use, the support is parallel to and extends along the side of the casing, and the antenna is near the lower end of the side of the casing. When in use, the support is pivoted by the user so that it is substantially in alignment with the casing.
WO-A-94 21053 proposes to dispose the antenna at the bottom end of a portable telephone. To this end, in one embodiment, it proposes to use a telescopic antenna which is directed downwards and whose end projects beyond the casing near the microphone, which is in the lower part of the casing. In a different embodiment, the invention proposes a fixing for adapting the same solution to existing casings which have an antenna in the upper part of the casing. To this end, it proposes to use an antenna extension which is connected to the base of the antenna at the upper end of the casing and extends along the side or the back of the casing. The extension is screened and contains a telescopic folding antenna which extends downwards when in use.
JP-A-7022832 proposes a reverse F type antenna in the form of a quarter-wave microstrip antenna which has a resonant frequency slightly different from that of the main antenna to enable the system to operate over a wider band and to limit gain reduction.
The above prior art solutions address only the electromagnetic radiation from the antenna and the distance between the radiating part of the antenna and the user""s head. They do not address the currents which flow when in use in the casing and which degrade the link. The inventors have found that in the proposed solutions the efficiency of the antenna is increased because attenuation by the user""s head is reduced. This leads to a corresponding increase in the currents or voltages in the casing. Radiation towards the user""s head is therefore increased, in particular if the user""s hand does not sufficiently attenuate the currents in the casing. This further reduces the efficiency of the link.
The invention proposes a solution to this new problem of indirect radiation towards the user""s head via the casing. It addresses not only the radiation induced directly by the radiating part of the antenna but also that induced indirectly by the currents in the casing. In this way it improves the efficiency of the link.
To be more precise, the invention proposes a transceiver including a casing, an antenna support mounted on the casing at one end and an antenna mounted on the other end of the antenna support, which has a length substantially equal to one quarter of the wavelength of waves emitted by the antenna.
In one embodiment of the invention said length is the physical length of the support.
In another embodiment of the invention the support is provided with reactances and the electrical length of the support corresponds to one quarter of the wavelength of waves emitted by the antenna.
The support is advantageously provided with lumped reactances and said electrical length of the support corresponds to one quarter of the wavelength of waves emitted by the antenna in at least two frequency bands.
The casing is preferably a metal or screened casing.
In an embodiment of the invention the antenna support is parallel to one side of the casing.
In another embodiment of the invention the casing includes a loudspeaker and the support is disposed along the side of the casing opposite the loudspeaker.
The support is preferably in the form of a coaxial cable.
The transceiver advantageously further includes an outer casing enclosing the casing and the antenna support.
Finally, the transceiver preferably constitutes a telephone for mobile networks.